The present invention relates to a green light emitting phosphor, particularly a green light emitting phosphor of high excitation, which can be applied to, for example, a projector tube or a high-brightness phosphor display tube.
Among the known Tb (terbium) activated green light emitting phosphors, the phosphor Y.sub.3 (Al, Ga).sub.5 O.sub.12 :Tb provides excellent brightness and saturation characteristic through the addition of Ga (gallium) as a green light emitting phosphor which is excited by electron beams of high energy. This is disclosed, for example, in Japanese Laid-Open Patent Gazette No. 60-101175.
A disadvantage of this known green light emitting phosphor is that it will deteriorate through the irradiation of electron beams. In other words, the body color of the green light emitting phosphor is changed by a so-called "burning". The burning occurs when the green light emitting phosphor is heated by the irradiation of electron beams. For example, when a phosphor screen of a projector tube or of a high-brightness phosphor display tube is formed using this type of known phosphor, and electron beams continuously impinge on the phosphor screen, in order to display a still image or a still picture portion of an image, the "electron beam burning" occurs locally, and deteriorates the image quality.
FIG. 1 shows the measured results of relative brightness versus aging time of phosphor screens. In the example used with this figure, the phosphor screens for an 8-inch projector tube were formed of green light emitting phosphors, Y.sub.3 Al.sub.x Ga.sub.5-x O.sub.12 :Tb of 7 mol percent, and the amount of Ga was changed, respectively. These phosphors were continuously irradiated with electron beams under conditions where the phosphor screen voltage (anode voltage, i.e. excitation voltage) was 32 kV and the anode current was 5 .mu.A/Cm.sup.2. An initial brightness was taken as a reference (100%).
In FIG. 1, an open square, an open circle, and an open triangle chart the measured results of the relative brightness of the green light emitting phosphors where x=0, 3, and 5 in Y.sub.3 Al.sub.x Ga.sub.5-x O.sub.12 :Tb, respectively. When comparing curves 1, 2, and 3, which follow from the measured results, it is shown that as the amount of Ga increases, the amount of electron beam burning, i.e. the amount of deterioration of brightness of the green light emitting phosphor by the irradiation of electron beams, is increased. Although the brightness and saturation characteristics of the green light emitting phosphor are improved by the substitution of Ga, it is unavoidable that the irradiation of electron beams will cause the brightness of the green light emitting phosphor to deteriorate.
One of the reasons for the deterioration of the brightness of the green light emitting phosphor is the valence state of Tb. Tb enters this type of phosphor at a so-called Y site, in the form of a trivalent. Unfortunately, Tb is in a stable condition when it is in the form of a tetravelent. If the phosphor is irradiated by electron beams, Tb's valence state can easily shift to that of a tetravelent element which causes the body color of the green light emitting phosphor to become brownish. In addition, Tb enters the Y site in an unstable state which causes a deterioration of the brightness characteristic of the green light emitting phosphor.